Traditionally, wire bonding is used to connect input/output (I/O) terminals (i.e., connection points) of a die to a package. More recently, under bump metallization (UBM) has been used as I/O terminals to connect the die to a packaging substrate. Traditional UBMs have a circular cross-section shape and provide several advantages over traditional wire bonding. UBMs are smaller than wire bonding, thus a die can have more I/O terminals by using UBMs than wire bonding. Therefore, UBMs can effectively increase the density of I/O terminals on a die. In addition, since UBMs are direct and short connections between the die and the packaging substrate, UBMs have higher speed performance relative to wire bonding.
A typical die is fabricated by depositing several metal and dielectric layers on top of a substrate. The substrate, metal layers and dielectric layers are what form the circuit elements of the die. The process of fabricating UBMs consists of depositing a metallization layer on top of a conventional top layer of the die. This metallization layer is what forms the UBMs. Solder is then deposited on the UBMs. Once the solder is deposited, the die is flipped and connected to a packaging substrate by connecting the solder to traces on the packaging substrate.
FIG. 1 illustrates a portion of a die that includes a UBM. As shown in FIG. 1, the portion of the die 100 includes a substrate 102, several metal and dielectric layers 104, a passivation layer 106, a UBM 108 and a solder 110. The substrate 102, metal and dielectric layers 104 form the circuit elements of the die 100. The passivation layer 106 is a protective layer of the die 100. The passivation layer 106 is a dielectric that is deposited over the last metal layer of the die 100. As shown in FIG. 1, the UBM 108 is fabricated on top of an opening in the passivation layer 106. The opening is created by etching the passivation layer 106. This opening in the passivation layer 106, once filled, becomes part of the UBM 108. Although not shown in FIG. 1, the UBM 108 has a circular cross-section. Once the UBM 108 is fabricated, a solder 110 is deposited on top of the UBM 108. In some implementations, the UBM 108 and the solder 110 are collectively referred to as a bump. Once this process is complete, the die 100 is flipped upside down and connected to a packaging substrate (not shown).
FIG. 2 illustrates an example of a portion of a die being connected to a portion of a packaging substrate. As shown in FIG. 2, the die 100 of FIG. 1 has been flipped and is being connected to a packaging substrate 200 that includes a trace 202. FIG. 2 illustrates that the solder 110 is being connected to the trace 202 of the packaging substrate 200. FIG. 2 only shows one solder being connected to a trace. However, in other instances, a die will have many UBMs and solders and a packaging substrate will have many traces. In such instances, each solder on the die will be connected to a respective trace on the packaging substrate.
One major drawback of connecting the die to the packaging substrate using this process is that a lot of stress (e.g., thermal stress, mechanical stress) is applied to the die. Thermal stress is the result of the substrate of the die having a different coefficient of thermal expansion than the coefficient of thermal expansion of the packaging substrate. Thus, the substrate of the die and the packaging substrate will expand or contract differently based on its respective coefficient of thermal expansion. The differences in expansion and contraction between the two substrates causes stress to be applied on the other components of the die and the package, including the metal layers, the dielectric layers, the passivation layer, the UBMs, the solders and the traces. The metal layers, the dielectric layers and the passivation layer are especially susceptible to stress. In particular, low K (LK) dielectrics or extremely low K (ELK) dielectrics tend to be brittle and can easily crack under stress.
Typically, large UBMs will absorb much of the stress between the two substrates, thereby reducing the likelihood of the cracking and/or delamination of the die. However, there is a trend towards the UBMs getting smaller and smaller in order to put as many UBMs on a die as possible while at the same time satisfying minimum pitch requirements between traces on the packaging substrate. With smaller size UBMs, this invariably leads to more stress being applied on other parts of the die, including the metal layers, the dielectric layers and the passivation layer. This can result in the cracking and/or delamination of the die, which ultimately results in a defective die.
As mentioned above, the density or number of UBMs on a die is limited by minimum pitch requirements. FIG. 3 illustrates examples of minimum pitch requirements. A pitch is defined as the center-to-center distance between two elements/structures on a die and/or package. For example, a pitch may be defined as the center-to-center distance between two neighboring traces or two neighboring UBMs. FIG. 3 illustrates a longitudinal pitch between a first UBM 300 and a second UBM 302. FIG. 3 also illustrates a lateral pitch between the second UBM 302 and a third UBM 304. These longitudinal and lateral pitches may be minimum pitch requirements for the die and/or substrate package. Minimum pitch requirements are required in a die and/or substrate to insure that there is enough spacing between elements/structures. This is done to reduce or avoid crosstalk that may occur when elements/structures are too close to each other. Minimum pitch requirements may also be required because of concerns for shorting and dielectric breakdown. FIG. 3 also illustrates a bump to trace clearance between the UBM 304 and the trace 306. This clearance specifies the minimum distance between the edge of a UBM and the edge of a trace in order to avoid or reduce crosstalk. Thus, there are dueling concerns when designing a UBM, on one hand, the UBMs have to be large enough in order to absorb the amount of stress that is put on a die, but at the same time the UBMs have to be small enough to meet minimum pitch requirements.
Therefore, there is a need for an improved UBM design that minimizes the likelihood of the die cracking and/or die delamination when the die is coupled to a semiconductor package.